M 31, also known as Andromeda galaxy or nebula, is the largest spiral galaxy of the local group, also including our Milky Way. The distance is about 2.5 million light years, its diameter about 142,000 light years. It is estimated that it contains about a trillion stars. Cosmological models assume that the Milky Way and M31 will merge in about 4-6 billion years and form a giant elliptical galaxy.